The present invention relates to semiconductor variable optical attenuators, and more particularly relates to calibrating and operating a semiconductor variable optical attenuator while factoring in changes in temperature.
Variable optical attenuators (VOA) are utilized widely throughout optical communications networks. The primary function of a VOA is to control the optical power propagating within an optical fiber. VOAs have been developed with a variety of technologies. Some forms of VOA involve moving parts (spinning filter wheels or fiber flexure to induce controlled loss). Other technologies include electro-optic devices that do not require moving parts, for example, polarization rotation in an electro-optic crystal followed by a polarizer. Semiconductor VOAs fall into the latter category of electro-optic VOAs, for which no moving parts are necessary. In general, an electric field is applied to the semiconductor material, which by virtue of the Franz-Keldysh effect in bulk semiconductors, or the quantum-confined Stark effect in quantum well materials, moves the band gap of the material. The band gap can be illustrated as an opening allowing the passage of photons therethrough. A wider gap allows a lot of photons to pass through, while a narrow gap attenuates the amount of photons, and thus the optical signal, passing therethrough. A light wave propagating at a wavelength nearby to a band edge, but just under the band gap, can thus be exposed to a variable and voltage controllable attenuation.
Semiconductor VOAs are relatively small and can be integrated into other semiconductor opto-electronic components. Semiconductor VOAs can be manufactured in high volumes using standard semiconductor processing methods. Semiconductor VOAs have no moving parts, and thus tend to have long term reliability. The semiconductor material that forms the semiconductor VOA is neither piezoelectric, nor pyroelectric. Therefore, internal fields do not build up with stress or temperature, which might otherwise cause the attenuation level to change over time. At a fixed or known temperature, the semiconductor VOA attenuation level is highly repeatable.
However, the degree of attenuation produced by the semiconductor VOA is temperature dependent. The band gap (position of band edge) is sensitive to an applied electric field, and it is also sensitive to its absolute temperature. The variation of attenuation level can be substantial even over a few degrees of temperature variation. An optical tap (power monitor) is generally employed after the semiconductor VOA to measure and control the VOA attenuation level to maintain a constant known output power.
There is a need for a system and method for monitoring and controlling the attenuation level of the semiconductor VOA relative to an absolute temperature of the VOA without the use of a power monitor. The present invention is directed toward further solutions to address this need.
In accordance with one example embodiment of the present invention, a method of calibrating a VOA includes the step of heating or cooling the VOA to achieve a first constant temperature. The voltage is varied to the VOA, which correspondingly varies the attenuation level of the VOA. The attenuation levels supplied by the VOA are measured at different voltages and a first constant temperature of the VOA. The VOA is then heated or cooled to achieve a second constant temperature, and the steps of varying the voltage and measuring the attenuation levels for the second constant temperature are repeated.
In accordance with further aspects of the present invention, the voltage and attenuation levels are recorded. A calibration reference is then generated based on the voltage and attenuation levels. The calibration reference can take the form of at least one lookup table, at least one plotted curve, at least one data set, or at least one empirical equation.
According to further aspects of the present invention, an interpolation reference expressing a relationship between the temperature and attenuation levels for fixed voltage can be generated. The interpolation reference can take the form of at least one lookup table, at least one plotted curve, at least one data set, or at least one empirical equation.
According to further aspects of the present invention, an interpolation reference expressing a relationship between the temperature and voltage levels for fixed attenuation can be generated. The interpolation reference can take the form of at least one lookup table, at least one plotted curve, at least one data set, or at least one empirical equation.
In accordance with another embodiment of the present invention, the VOA can be disposed on a temperature controllable surface. The step of heating and cooling the VOA to achieve the first constant temperature and the second constant temperature can include varying the temperature of the temperature controllable surface to result in one of the first and second constant temperatures. In addition, the step of varying the voltage can include sending a digital instruction signal to a digital to analog converter and converting the digital instruction signal to an analog voltage output.
In accordance with still another example embodiment of the present invention, a method for attenuating an optical signal includes the step of providing a variable optical attenuator for attenuating the optical signal. The variable optical attenuator is instructed to maintain the desired attenuation level of the optical signal. The temperature of the variable optical attenuator is periodically sensed, and a required voltage level is determined to achieve the desired attenuation level based at least partially on the periodically sensed temperature of the VOA. The method can further include the step of increasing and decreasing a voltage to the VOA to achieve the required voltage level and thus the desired attenuation level.
The provision of a VOA can include the step of disposing a semiconductor type VOA on a thermal electric cooler. The thermal electric cooler represents a means for controlling the temperature of the semiconductor type VOA.
According to one aspect of the present invention, the instructing step can include delivering a predetermined electric field to the semiconductor type VOA to effect the attenuation level.
According to further aspects of the present invention, the step of providing the optical signal can be executed by powering a diode laser source, although a diode laser is not the only potential source for the optical signal. The step of fine tuning a wavelength of the optical signal can be accomplished by adjusting the temperature of the diode laser source. Each of the sensing and determining steps can occur on a periodic basis. A temperature sensor can be placed relative to the VOA to measure the temperature of the VOA.
In accordance with further aspects of the present invention, the voltage level determination can be carried out by looking up voltage levels on a lookup table based on the temperature of the VOA and desired attenuation level. The voltage level can be measured and compared with the required voltage level according to the lookup table.
Alternatively, the required voltage level can be determined by executing an algorithm to calculate the required voltage level based on the temperature of the VOA and the desired attenuation level. The determination of the required voltage level can include interpolating between calculated values of required voltage levels based on the temperature of the VOA and the desired attenuation level.
In accordance with yet another example embodiment of the present invention, a system is provided for emitting an optical signal. The system includes a temperature controllable surface. An optical signal source is disposed on the temperature controllable surface. A plurality of lenses are disposed in line with the optical path of the optical signal source. A VOA is disposed in line with the optical path of the optical signal source on the temperature controllable surface, and a temperature sensor is disposed to sense the temperature of the VOA.